An integrated circuit (IC) is a combination of interconnected circuit elements inseparably associated on or within a continuous substrate, the substrate being the supporting material upon which and/or within which an integrated circuit is fabricated. Generally an integrated circuit is fabricated within and/or on a chip of semiconductor material, usually silicon, with the resistors, capacitors, diodes, transistors, etc. (as required) built into and/or on the chip. The semiconductor body is either a single crystal material or single crystal islands in a polycrystalline material, depending on the method for electrical isolation of the circuit components.
A thyristor is a non-linear, solid state device that is bistable; that is, it has both a high and a low impedance state. Typically a thyristor has a four layer PNPN structure. It is usually switched from one impedance state to the other by means of a control of gating signal applied to one of the base regions. A reverse switching rectifier is a two-terminal thyristor having a rectifier with active regions in common with the base regions and circuited in reverse parallel in the same semiconductor body, see U.S. Pat. No. 3,584,270 and Ankrum, Semiconductor Electronics, p. 535 (1971). A reverse conducting thyristor is a three-terminal thyristor having a rectifier with active regions in common with the base regions and circuited in reverse parallel in the same semiconductor body, see Kokosa, IEEE Trans Electron Devices, ED-17, 669 (1970). These devices have been limited in their turn-off capability, particularly in high conduction states, by reason of their common active regions. That is, when either the equivalent thyristor or rectifier was in the conducting state, the other device could not assume a blocking state before the excess electron-hole carriers were removed from the common base regions. Thus, the device was limited in its applications to slow turn-off applications.
A solid-state ac switch is a bidirectional thyristor. The most common of these thyristors are the "diac" which is a two-terminal switch, and the "triac" which is a three-terminal switch wherein one of the terminals is a gate electrode. Such bidirectional thyristors are mutilayer structures which have the equivalence of two thyristors in a single body, circuited in reverse parallel, see Ankrum, Semiconductor Electronics, pp. 531-32 (1971). These devices are limited in their turn-off capability by reason of the common active regions of the equivalent thyristors. During turn-off, these regions must be in a conduction state on one half of the ac cycle and in a blocking state on the other half of the cycle; and the blocking state cannot be normally established in one of the thyristors until the excess electron-hole carriers are removed from the common regions of the device.
To illustrate, most uses of triacs are in resistive-inductive (R-L) circuits where current and voltage are not zero simultaneously. Rather, the initially conducting portion of the triac will conduct into the period of time when the load voltage across the triac has reversed polarity. When formation of a blocking mode is attempted, the load voltage is rising across the device in the direction opposite to the voltage that caused the previous conducting mode. To avoid triggering an immediate conducting mode in the forward biased non-conducting portion of the triac, the triac must have a certain tolerance to this rate of rise of load voltage (dV/dt). This tolerance to rate of rise of load voltage is termed "non-commutating dV/dt" or simply "dV/dt.sub.c ". This parameter is smaller than the dV/dt of a thyristor on forward biasing because of the common active regions of the two thyristor equivalents of the triac.
It has been proposed to restrict the commutating conductance between the adjacent devices through the common active regions by selective diffusion of gold into the common regions between the devices. See, e.g., U.S. Pat. No. 3,727,116. But this proposal has not provided a satisfactory solution. The gold diffusion increases the recombination centers to provide dynamic isolation between the devices; however, the gold diffusion substantially increases the carrier regeneration rate and correspondingly causes high leakage current that can kill the voltage rating of the device. Further, the gold easily diffuses laterally into the active regions of the devices resulting in limited high current and high frequency operation.
Because of such turn-off limitations, bilateral thyristors, reverse switching rectifiers and reverse conducting thyristors have been limited in their use. Rather, separated discrete thyristors and diodes are wired in reverse parallel to provide fast turn-off in fast switching and high frequency applications. These discrete component circuits do not suffer from the disadvantages characteristic of the integrated circuit devices, but the cost and space savings and reliability attributed to integrated circuits is lost.